Clean air system

ABSTRACT

The present invention provides a method for controlling exhaust gas emissions including platinum, wherein exhaust gas is brought into contact with an emissions control catalyst and the oxygen content of the exhaust gas is controlled in order to maintain fuel rich stoichiometry close to the stoichiometric value.

FIELD OF THE INVENTION

The present invention is generally directed to a method for achievingpollutant free operation of automotive engines and other sources ofcarbonaceous and nitric oxide pollutants. More specifically, the presentinvention is directed at controlling emissions of platinum oxides fromcatalytic converters.

BACKGROUND OF THE INVENTION

Historically, great strides have been made in controlling automotiveemissions. Early oxidation catalyst systems were very effective incontrolling hydrocarbon and carbon monoxide emissions. However, it wassoon recognized that control of nitrogen oxides (NOx) was alsonecessary. Because NOx control catalysts are typically ineffective underoxidizing conditions, two stage systems were used having a reducingfirst stage followed by an oxidizing second stage with added air. Amajor improvement was the development of three way catalysts and thesystem of dithering the fuel air ratio between rich and lean around thestoichiometric value. This allows both NOx reduction and oxidation ofhydrocarbons and carbon monoxide, and has been the standard approach formany years. All these approaches have a serious flaw. Any period of leanoperation will allow loss of platinum as platinum oxide. Thus it shouldbe no surprise that European researchers have recently discoveredwidespread platinum contamination of the environment. This is apotentially serious problem since workers in a platinum refinery havedeveloped serious allergic reaction to extremely low concentrations.Although a guard bed could be used to gather platinum, frequentreplacement would likely be required.

Based on the foregoing, it is an objective of the present invention toprovide a method and apparatus to eliminate loss of platinum fromcatalytic converters without impairing effectiveness.

SUMMARY OF THE INVENTION

The present invention comprises a method and apparatus for operating acatalytic converter system. In the method of this invention, a catalyticconverter, such as used on present day automobiles, is operated on thefuel rich side of stoichiometric but with sufficient air to consume mostof the hydrocarbons and carbon monoxide in the exhaust. It has now beenfound that as low or lower HC, CO and NOx emissions can be achievedusing the method of the present invention as with dithering systems.Non-methane hydrocarbon emissions of less than five ppm are achievablewith carbon monoxide representing the bulk of residual fuel values. Inthe absence of excess oxygen on the catalyst surface, platinum loss isnegligible. Even methane is readily converted in a fuel richenvironment. It has now been shown that in fuel rich oxidation ofmethane, no measurable loss of platinum could be detected after morethan 2000 hours of operation. In contrast, nearly all the platinum waslost in less than 200 hours running fuel lean.

It has also been found that minor platinum emissions resulting from anyfailure of the control system can be captured by use of a guard bed of amaterial such as alumina down stream of the catalytic reactor. Note thatavailable modern digital control systems have proven highly reliable.

In the present invention, the required control of exhaust gasstoichiometry is achievable such as by feedback of measured values froma universal oxygen sensor to a modern engine control computer. Test runshave demonstrated that high conversion levels of all three pollutantsare achievable. High mass transfer efficiency catalysts, such as theshort channel length catalysts of U.S. Pat. No. 5,051,241, allow highconversion of hydrocarbons and carbon monoxide even with low residualoxygen levels. Advantageously, stoichiometry is controlled to supplysufficient oxygen to convert all but ten to two hundred ppm of the fuelvalues present.

While the present invention has been described in detail with referenceto providing a method for eliminating the loss of platinum fromcatalytic converters, other catalytic reactor configurations exhibitingthe characteristics taught herein are contemplated. Therefore, thespirit and scope of the invention should not be limited to thedescription of the preferred embodiments described herein.

1. A method for controlling exhaust gas emissions including platinum, comprising: a) contacting the exhaust gas with an emissions control catalyst; and b) controlling the oxygen content of the exhaust gas to maintain fuel rich stoichiometry close to the stoichiometric value.
 2. The method of claim 1 wherein the catalyst is a three way catalyst.
 3. The method of claim 1 wherein the catalyst is an oxidation catalyst.
 4. The method of claim 1 wherein a universal oxygen sensor is used to measure exhaust gas stoichiometry.
 5. The method of claim 4 wherein the measured exhaust gas stoichiometry is fed real time to a fuel flow control computer.
 6. The method of claim 5 wherein the fuel control computer is programmed to maintain a constant fuel-air stoichiometry in transient operation.
 7. The method of claim 6 wherein sufficient oxygen is present to consume all but between about ten and about two hundred ppm of the fuel values present.
 8. A method for operating an automotive engine comprising: a) controlling fuel flow in relation to air flow; b) measuring exhaust gas stoichiometry using a universal oxygen sensor; and c) adjusting fuel flow to maintain exhaust gas stoichiometry slightly on the rich side of stoichiometric. 